Project Abstract: Merkel cell polyomavirus (MCV) is a small DNA tumor virus that causes approximately 80% of Merkel cell carcinoma (MCC), a highly aggressive skin cancer. Since the discovery of MCV in 2008, it has been shown that the MCV tumor antigens small T antigen (ST) and large T antigen (LT) are the only viral proteins consistently expressed in virus-positive MCC cells. Expression of ST alone in Rat1 cells was sufficient to induce cellular transformation suggesting that ST is the primary driver of cellular transformation in MCC development. Our lab determined that ST forms a specific complex with L-Myc/MAX and the Tip60/p400 complex (SLT complex) and that a ST mutant incapable of binding to L-Myc and the Tip60/p400 complex cannot transform cells. The Tip60/p400 complex is a large multi-subunit complex that has lysine acetyltransferase activity (Tip60 protein) and nucleosome exchange activity (p400 protein). Both of these activities have been correlated with active gene transcription but further work is necessary to fully understand the relationship between Tip60/p400 complex activity and gene expression. We hypothesize that MCV ST redirects the Tip60/p400 complex to L-Myc binding sites to activate a transcriptional program requiring their lysine acetyltransferase and histone remodeling activity to drive cellular transformation. In Aim 1, we will assess the effect of ST on Tip60/p400 complex binding and we will compare the direct targets of the Tip60/p400 complex in control cells with cells expressing ST to determine if ST redirects the complex from ?normal? Tip60/p400 complex binding sites to SLT target gene promoters to activate SLT target gene transcription. By comparing the list of genes that are direct targets of the SLT complex in normal human cells to those we previously identified in virus-positive MCC cells, we will generate a list of genes that are consistently regulated by ST. In Aim 2, we will assess the contribution of Tip60 histone acetyltransferase (HAT) and p400 nucleosome remodeling activities to ST-mediated gene expression changes and cellular transformation. In Aim 3, we will adapt the Fkbp/Frb inducible recruitment for epigenetic editing by Cas9 (FIRE- Cas9) method to the Tip60/p400 complex and then use this system to recruit the Tip60/p400 complex to SLT target genes in the absence of ST. Upon recruitment, we will measure changes to ST target gene transcription and the level of Tip60/p400-specific histone marks at SLT target gene promoters. Insights gained from investigating the ST interaction with L-Myc and the Tip60/p400 complex will provide a deeper understanding of the oncogenic roles of these factors and understanding the activities of the SLT complex will lead to identification of improved targeted therapies to combat MCC.